1. Field of the Invention
This invention relates to a process for preparing an organic polyisocyanate composition. The organic polyisocyanate composition herein means a prepolymer formed from at least two molecules of an aliphatic, alicyclic or araliphatic organic diisocyanate monomer. The prepolymer is generally an oligomer such as a dimer, a trimer, a tetramer and a pentamer or a mixture thereof. An araliphatic organic diisocyanate monomer means an isocyanate compound comprising an aromatic group and an aliphatic group where isocyanate groups are bonded to the aliphatic group.
2. Description of the Prior Art
A polyisocyanate composition having, on an average, at least two isocyanate groups in one molecule which have reactivity with active hydrogen-containing compounds, is widely used as a starting material for preparing polyurethanes in various fields such as foam materials, coating materials, adhesives, etc. by utilization of this property. Particularly, when this composition is used as the isocyanate component in a two component type polyurethane coating material, there can be obtained a coated film having excellent coating properties such as abrasion resistance, adhesion, chemical resistance, solvent resistance, flexibility, electrical insulation, etc. Such a coating material is highly evaluated as one of the most excellent coating materials.
However, the polyisocyanate composition is required to have various characteristic features depending on the field of uses and regions for applications. For example, in some cases it is desired that quick dryability is imparted to a formulation containing the polyisocyanate composition, while other properties such as elasticity, strength, hardness and weatherability of the polyurethane obtained must be improved in other cases. Thus, disadvantageously such requirements cannot be satisfied with one to several kinds of polyisocyanates. Accordingly, it is a general practice to blend several kinds of polyisocyanate compositions in order to obtain desirable characteristic features. But there is required a high level of technique and experience for designing an appropriate formulation of compositions and it is also very cumbersome to get all of these polyisocyanate compositions ready. Thus, the above described problem has not been fundamentally solved.
Generally speaking, as a suitable isocyanate component to be used as a hardener for polyurethane coating materials, foam materials and adhesives, dissocyanate monomers as such cannot be used due to their toxicity and stimulation since they have a relatively higher vapor pressure and accordingly, they are modified into polymers of the diisocyanate monomers, i.e., polyisocyanate compositions to remove the toxicity and stimulation before use.
On the other hand, in various fields including the field of coating materials, there is a strong demand for a high solids formulation such as coating materials of high solids content where the amount of a thinner is reduced to a minimum from the viewpoint of environmental safety, energy-saving for drying and cost-saving of the thinner. But the viscosity of the above described polyisocyanate compositions which can be used as a hardener for polyurethane coating materials is generally very high and some polyisocyanate compositions are solid at normal temperature. For convenience in usage, such highly viscous or solid polyisocyanates compositions can only be used by being diluted with a solvent such as ethyl acetate, butyl acetate, cellosolve acetate, toluene, xylene, etc. This high viscosity of the polyisocyanate compositions is a great obstacle to the preparation of a high solids formulation of polyurethane coating materials. Thus, in this field it is fully expected to provide polyisocyanate compositions having a low viscosity in which the non-volatiles content is even 100%, hence requiring no thinner in the preparation of coating materials, and also not having toxicity and irritant odor.
U.S. Pat. No. 3,903,127 describes the preparation of polyisocyanates with a biuret structure suitable as an isocyanate component for high solids content urethane coating materials by reacting a diisocyanate with a biuretizing agent in proportions corresponding to a mol ratio of the diisocyanate to the biuretizing agent of at least 11:1. Particularly as biuret polyisocyanates obtained from hexamethylene diisocyanate as the starting material, there are disclosed polyisocyanate compositions having a comparatively low viscosity of about 700 to about 800 centipoise [hereinafter "cP"] at 25.degree. C.
In order to obtain a hexamethylene diisocyanate biuret polyisocyanate composition having a low viscosity of about 800 cP at 25.degree. C., however, it is necessary to control the mol ratio of hexamethylene diisocyanate to the biuretizing agent to an extremely great ratio of 30:1 to 50:1, and a markedly large amount of unreacted monomer must be removed and recovered before the polyisocyanate composition formed is separated. This results in an increase in the commercial production cost of a single species of polyisocyanate composition. Furthermore, it is disadvantageous to commercially produce a number of various polyisocyanate compositions corresponding to various properties such as the viscosity and the dryability of compositions containing the polyisocyanate compositions required in various fields of use and application as well as corresponding to the final polyurethane products.
As the result of extensive investigations directed to very easily and economically preparing an organic polyisocyanate composition which is further decreased in viscosity and/or improved in compliance to various requirements with respect to the properties such as the dryability of a composition containing the organic polyisocyanate composition as well as the properties of final polyurethane products, it has now been found that an aliphatic, alicyclic or araliphatic organic polyisocyanate composition can be decreased in its viscosity and/or can be modified by heating at a specified temperature in the presence of a specified amount of an aliphatic, alicyclic or araliphatic organic diisocyanate monomer.